(1) Field of the Invention
This invention relates to a method for washing high molecular substances stuck on the interior of a production apparatus or molding apparatus for an aromatic vinyl-acrylonitrile copolymer as well as a method for regenerating and recovering the solvent which has been used in the above washing method.
(2) Description of the Prior Art
It has been known that, upon producing or molding an aromatic vinyl-acrylonitrile copolymer, high molecular substances which are insoluble or hardly soluble in the monomers or polymerization solvent generally stick on the interior of the production apparatus or molding machine.
These high molecular substances principally occur in reaction vessels and heat transfer devices, thereby lowering their heat transfer capacity and inducing pipe clogging and creating serious obstacles for steady operation due to lowered controllability of reactions and lowered productivity. Furthermore, parts of the thus-stuck high molecular substances mix in the products and damage their appearance and hues. Under the circumstances, it is necessary to interrupt the operation frequently so as to remove high molecular substances which have stuck on apparatus.
The removal of the above-mentioned high molecular substances may be effected manually. However, this method is accompanied by such drawbacks that it not only requires lots of man power and working time but also induces breakage of apparatus and contamination of workers due to chemical substances remaining in the apparatus. Alternatively, the thus-stuck high molecular substances may be peeled off and removed by means of a high pressure fluid. This method may however not be relied upon if the apparatus includes spots where the high pressure fluid does not have any direct access due to the structure of the apparatus or if the apparatus is not strong enough to apply this method.
In order to avoid the above-mentioned drawbacks, washing is usually carried out with a solvent. Here, it is necessary to employ a solvent which is capable of dissolving polymers, because a monomeric solvent has poor solubility at low temperatures and, at high temperatures, it then undergoes polymerization. For example, alkylamides such as dimethylformamide and alkylsulfoxides such as dimethylsulfoxide are known to be effective solvents to aromatic vinyl-acrylonitrile copolymers. However, some of cross-linked or modified high molecular substances may still not be dissolved in such solvents and remain in the apparatus. Such undissolved high molecular substances serve as nuclei and induce deposition of fresh high molecular substances thereon. Moreover, high molecular substances, which have not been completely dissolved and removed, are discolored under the influence of the solvent and heat. If they are caused to mix in products, their quality, notably, their hues will be damaged.
As a still alternative washing method, Japanese patent publication No. 8087/1980 discloses to dissolve and remove gelated substances, which occur upon producing high-nitrile vinyl resins containing acrylonitrile as much as at least 50 mole %, using a solution which contains an alkali metal, alkaline earth metal and the like. The dissolution capacity of such a solution has, however, been found insufficient when it is applied to high molecular substances occurred upon producing low-nitrile vinyl resins.
Generally speaking, the high molecular substances of the above type are in a swollen state due to inclusion of monomers for aromatic vinyl-acrylonitrile copolymers. They thus contain lots of monomers. Due to such monomers, when washing such high molecular substances with a solvent, the monomers are caused to mix in the solvent and reduce the washing capacity of the solvent. In some instances, the capacity of washing high molecular substances may reach its lower limit even after a single washing operation. For instance, when producing a styrene-acrylonitrile copolymer by the continuous bulk polymerization method, high molecular substances and monomers are present in high concentrations in the polymerization vessel upon completion of the polymerization. In the case of washing the polymerization vessel with a washing solvent in accordance with the batch method, the concentration of the monomers and the like in the resultant washing reaches 2-20 wt. %, or normally 6-12 wt. %. Thus, the washing is unsuitable for its reutilization as a washing solvent. In this case, it is necessary to recover and purify the washing solvent. The interior of a polymerization vessel may be dried to drive off monomers with a view toward avoiding any substantial concentration increase of the monomers in the washing solvent. However, further problems arise from this method, because it requires a long drying period of time and it is difficult for workers to carry out.
Since washing solvents are generally expensive, it is necessary to recover and reuse them. The recovery of such solvents cannot be effected by the filtration method or absorption method, because high molecular substances and monomers have to be separated therefrom. The routinely-employed recovery procedure makes use of a distillation treatment. Due to the presence of high molecular substances, the washing solvent and monomers are collected as a fraction of distillate in accordance with the batch method and then high molecular substances are separated. However, such a batch-type distillation operation is accompanied by many drawbacks, since a large portion of the solvent is lost upon distilling monomers, the distillation operation is time-consuming, and the distillation apparatus becomes unavoidably large and large expenses will be required for the facilities when washings are treated in a large quantity.